The present invention relates to the electronic processing for aural reproduction of audio signals in electrical form.
In reproducing an audio recording, not only is the audio source reproduced, but also reproduced are undesirable extraneous noise components caused by imperfections in the original recording process, the storage medium, and in the reproduction process. Such noises commonly are designated by such terms as "surface noise", "crackle", "ticks", etc. These noises are generally composed of: (i) a component sometimes known as continuous or "white" noise which is of an amplitude generally not exceeding the desired signal amplitude and is distributed fairly uniformly throughout the audio frequency range; and (ii) a component which consists of transient noises generally of short duration and amplitudes at least comparable to that of the desired audio signal. (As used herein, the term "input signal" refers to the electrical signal, consisting of generally continuous and transient signals, bearing the audio information intended to be heard by the listener or receiver; the term "noise" refers to the noise, both continuous and transient, which may be imposed on the input signal; the term "total input signal" refers to the total electrical signal being transmitted, i.e., input signal plus noise; and the terms "audio transient" and "noise transient" refer, respectively, to those portions of the input signal and noise in transient form.)
The applicant is a joint inventor of systems for reducing transient noise in audio signals which are the subjects of U.S. Pat. Nos. 4,151,471 and 4,155,041, and of co-pending U.S. patent application Ser. No. 957,713 filed Nov. 6, 1978. The foregoing systems are concerned with suppressing noise in a total input signal that is caused by discrete noise transients. The present invention is principally concerned with the suppression of perceived continuous noise, although transient noise is also reduced by this invention.
In the processing of an electrical audio signal for aural reproduction by means of an audio amplifier, the concept of automatically varying the width of the frequency passband of the amplifier to reduce noise has long been known. The variation of the passband in relation to the amplitude of the total input signal in order to reduce the audibility of continuous noise with a minimum effect on the perceived fidelity of the input signal goes back at least as far as Harmon H. Scott's U.S. Pat. Nos. 2,606,969, 2,606,970, 2,606,971 and 2,606,973. Those patents provide an excellent account of the psychoacoustical basis of this principle. Fundamentally, the variation in signal bandwidth takes advantage of the variation in sensitivity of the human ear over the audio frequency range with the loudness of the sound received. Louder sounds tend to mask noise and permit use of a wider frequency range than do quieter sounds for the same listener-perceived tonal quality. Scott's "dynamic noise suppressor" operates in a rather gradual manner in varying the passband of the amplifier, especially after loud passages, lest a too evident change in noise become audible. The dynamic noise suppressor is incapable of supressing transient noises selectively. High amplitude noise transients can cause an increase in signal bandwidth that is audible after the passage of the transient. Low amplitude noise transients are only generally suppressed by the dynamic noise suppressor depending upon each transient's exact characteristics. Various other embodiments of this principle have been disclosed in U.S. Pat. Nos. 2,638,501 (Coleman) and 3,678,416 (Burwen).
Dynamic noise suppressors that operate by varying the extremes of the signal bandwidth are not effective in reducing noises that occur at or about the middle of the audio frequency range. To reduce noise in middle of the bandwidth, some disclosures, notably U.S. Pat. Nos.3,403,224 (Schroeder), 3,803,357 (Sacks) and 3,989,897 (Carver), divide the audio frequency range, or a significant portion of it, into a plurality of contiguous frequency bands of, typically, an octave in width. Various methods are disclosed in the cited patents for determining, over a given period of time, the relation of signal to noise in each of the frequency bands and disabling those bands from transmission found to contain chiefly noise components. The signal is reconstituted by the combining of the frequency bands not disabled.
Compressor-expander systems reduce noise in audio signals by compressing the dynamic range of the signal in the recording process, and by expanding it in the playback process to the extent that it had been compressed. Such systems are disclosed in U.S. Pat. Nos. 3,665,345 (Dolby), 3,729,693 (Dolby), 3,732,371 (Burwen), 3,813,559 (DeBoer), 3,815,039 (Fujisawa), 3,828,280 (Dolby), 3,829,715 (VanSluys) and 3,846,719 (Dolby). These systems are effective in reducing noise without audible effect on the perceived fidelity of reproduction only in those sound recordings which are specifically pre-encoded for such systems.